An example of a voltage controlled oscillator (VCO: Voltage Control Oscillator) includes a voltage control crystal oscillator (VCXO: Voltage Control Crystal Oscillator), and an output frequency of this VCXO is controlled by a supplied control voltage. For example, a VCXO used for a base station of mobile phones oscillates so as to be synchronized with a received GPS (Global Positioning System) signal. There is a case where some kind of cause makes it impossible to receive this GPS signal. In such a case, the VCXO is required to have superior holdover performance. That is, even if the GPS signal cannot be received, the VCXO is required to oscillate at a predetermined frequency and with high accuracy. In addition, it has been considered to enhance the communication speed between the mobile phone and the base station. In order to enhance the communication speed, it is required to ensure not only an increased frequency but also a high accuracy in a predetermined frequency. Due to these circumstances, it is also required that an output of the output frequency of the VCXO is controlled with higher accuracy.
Incidentally, at a production stage of the VCXO, each component that constitutes the VCXO is exchanged by a manufacturer while confirming an oscillation frequency actually output from the VCXO, and the components that ensure the desired output frequency are employed. However, after the components employed in such a manner are stored in a package, these components are possibly heated and deaerated with the package to prevent oxidation, or are possibly exposed to an inert gas atmosphere sealed in the package to prevent oxidation. That is, the respective components that constitute the VCXO are stored in different environments at the production stage and after the production. Even if the identical control voltage as the control voltage supplied at the production stage is supplied to the VCXO after the production, this may cause the output frequency to be lightly deviated from the frequency obtained at the production stage. Due to these circumstances, it is desired that the VCXO ensures adjusting an oscillation output finely after the production.
Japanese Unexamined Patent Application Publication No. 2012-138890 discloses a VCXO provided with a digital potentiometer in order to adjust an oscillation frequency. However, the number of taps of the digital potentiometer with the highest resolution is currently about 1024, and the only one digital potentiometer is included in this oscillator. In order to control the output frequency with high accuracy as described above, it is desired that a technique ensures adjusting the oscillation frequency more finely than a configuration of this Patent Literature 1 is required. While Japanese Unexamined Patent Application Publication No. 2011-171828 and Japanese Unexamined Patent Application Publication No. 2000-183650 also disclose a VCXO provided with a potentiometer, the VCXO cannot solve such a problem.
The disclosure has been made in view of the above-described problems, and it is an object of the disclosure to provide a voltage controlled oscillator that controls an output frequency with high accuracy.